rc16.el

Julian Assange (proff@iq.org)
Thu, 9 Apr 1998 04:55:37 +1000 (EST)

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;; who says you can't do stream cipher's in lisp?

;; rc16.el
;; 16bit ar-sea-four in elisp
;; this is GPL'd code.

;; most numbers in here are meant to be 16bits
(defconst xFFFF (- 65536 1))

;; rc16-context functions
(defun rc16-create-context ()
  (vector 0 0 (make-vector 65536 0)))

(defsubst rc16-context-x (context)
  (aref context 0))

(defsubst rc16-context-y (context)
  (aref context 1))

(defsubst rc16-context-state (context)
  (aref context 2))

;; sigh. why doesn't elisp have (setf (aref ...))?
(defsubst rc16-context-set-x (context val)
  (aset context 0 val))

(defsubst rc16-context-set-y (context val)
  (aset context 1 val))

;; set a key, init the context
(defun rc16-set-key (context key)
  (let ((state (rc16-context-state context)))
    ;; init context
    (rc16-context-set-x context 0)
    (rc16-context-set-y context 0)
    (let ((counter 0))
      (while (< counter 65536)
        (aset state counter counter)
        (setq counter (1+ counter))))

    ;; mix in key
    (let ((keyidx 0)
          (keylen (length key))
          (stateidx 0)
          (counter 0)
          temp1 temp2)
      (while (< counter 65536)
        (setq temp1 (aref state counter))
        (setq stateidx (logand (+ stateidx temp1 (aref key keyidx)) xFFFF))
        (setq temp2 (aref state stateidx))
        (aset state stateidx temp1)
        (aset state counter temp2)
        (setq keyidx (1+ keyidx))
        (if (= keyidx keylen) (setq keyidx 0))
        (setq counter (1+ counter))))))

(defun rc16-short (context)
  (let* ((state (rc16-context-state context))
         (x (logand (1+ (rc16-context-x context)) xFFFF))
         (sx (aref state x))
         (y (logand (+ sx (rc16-context-y context)) xFFFF))
         (sy (aref state y)) bytes-left))
      (if (= bytes-left 0)
          (progn
            (setq bytes-left 2)
            (setq rc16-val (rc16-short context))
            (setq hi-byte (ash rc16-val -8))
            (setq lo-byte (logand rc16-val 255)))
        (setq hi-byte lo-byte))
      (setq count (1+ count)))
    cipher))

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